Frequency discriminator



Jan. 8, 1946.

E. s. PURINGTON` FREQUENCY DISCRIMINATOR Filed May 27, 1942 4 Sheets-Sheet 1 Jan. 8, 1946.

E. s. PURINGTON 2,392,693 FREQUENCY DISCRIMINA'OR Filed May 27, 1942 4 Sheets-Sheet 3 lNvENToR ELLlsoN s. P RING-FON.

Jan. '8, 1946.

E. S. PURINGTON FREQUENCY DISCRIMINATOR Filed May "27, 1942 4 Sheets-Sheet 4 INVENTOR ING-ron. www 4 MEGACYCLES O 'iE Patented Jan. 8, 1946 FREQUENCY DISCRIMLN'ATOR Ellison S. Purington, Gloucester, Mass., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application May 27, 1942, Serial No. 444,630 l (Cl. Z50-8) 4 Claims.

This invention relates to radio kreceiving systems, and more Aparticularly to a system for reducing interference in radio reception.

The invention Afurther relates to receivers for 'receiving signals of the type Aproduced by the transmitter of the special system of radio telegraphic communication disclosed in U. S. Patent No. 1,690,719 granted to E. L. Chaffee and E. S. Purington on November 6, 1928. The essential features of that transmission system are:

' l. Creation of two or more continuous wave equivalents, usually by a direct or push-pull modulation process, involving an A oscillator, and a lower but superaudible frequency oscillator; the difference between the frequencies of the continuo us waves being designatedthe B frequency.

2. Articially frequency modulating the A oscillator, and consequently Wobbling all of the continuous wave equivalents, the frequencies of which depend upon the A oscillator.

3. Changing the difference or B frequency for purposes of signalling. When the high frequency energy is properly received and detected it will produce received currents of the B frequency which can be sharply tuned "to, the degree of sharpness possible being determined by the key speed of the signal being sent. The B frequency energy then operates a current detector, such as a heterodyne system, to make the conveyed signals intelligible.

The second process of articial wobble, or frequency modulation, primarily provides for a high degree of selectivity and reduction of fading. This frequency modulation is entirely articial and has nothing to do with the signal frequency. Its purpose is to spread the radiated energy over a wide band so that stray disturbances are reduced and intentional interferences from continuous Wave transmitters, spark and other I. C. W. transmitters can produce `interference for only a part of the time. In this way a diversified transmission is produced and since the rate of frequency modulation is chosen low, but with several excursions of modulation for a single dot or dash, any selective fading effects are averaged out very effectively. By the use of a suitable wobbler pattern and other features the system becomes private against reception by the usual types of receivers.

The third process of changing the spacing between the continuous wave equivalents without causing them to appear and disappear is primarily for maintenance of privacy. This is, also, a frequency modulation process at the signal rate usually applied to the B oscillator, and it is, there-` fore, possible to apply the well known principles of frequency modulation signalling in the receivers for such transmission systems. The present invention shows the details of one specific application, namely, the back to'back detector dis? closed in U. S. Patent No. 1,776,065 granted to E. L. Chaffee on September 16, 1930.

The present invention, also, consists in certain new and original features of construction and combinations of parts hereinafter set forth and claimed. Although th novel features which are believed to be characteristic of this invention will be particularly rpointed out in the claims appended hereto, the invention itself, as to its objects and advantages, the 'mode of its operation-and the manner of its organization, may be better understood by referring tothe followingv description taken in connection with the accompanying drawings forming a part thereof, 'in which: y

Figure 1 shows a schematic diagram of a transmitter used in connection with this invention,

Figure 2 illustrates schematically a receiver constructed in accordance with this invention,

' Figure 3 illustrates the continuous wave radiations as received by the receiver shown in Figure 2,

Figure 4 illustrates the output of the second detector of the receiver shown in Figure 2,

Figure 5 depicts an audio frequency discriminator used following the third ldetector of the receiving system of Figure 2,

Figure 6 shows a modified forml of 'part of the circuit illustrated in Figure 5,

Figure 7 shows a modification of the circuit of Figure 6,

Figure 8 illustrates an expander type compensator used as a modied form of Dart of the circuit shown in Figure 5,

Figure 9 illustrates the use of a limiter on the direct current output of the compensator of"Fig.`8,

Figure 10 shows a method of securing automatic volume control in accordance with total energy, as applied to the circuit of Fig. 5,

Figure 11 illustrates the transmitted spectrum of the transmitter illustrated in Figure 1.

Like reference characters denote similar circuit elements parts in the several figures of the drawings. In the following description and inthe claims parts will be identi'ed by specic names for convenience,-but they are intended to be as generic in their application to similar parts as the art will permit.l

' Referring to the accompanying drawings, and

more particularly to Figure 1, a transmitter is 'shown which may be used to produce the desired radiations. This transmitter may be constructed .in accordance with the teachings of the aforesaid U. s. Patent No. 1,690,719. The transmitter Vis schematically-shown as comprising an A wobnally generate oscillations of 50 kilocycles, is provided with a modulating signalby a key' arrangement, or B wobbler I 3, for changing the oscillator frequency between the values 49.95 and 50.05 kilocycles. The B oscillator is thus frequency modulated between these limits with a square wave signalling pattern.

It may, also, be said that the B oscillator generates two amplitude modulated waves, one of 49.95 kilocycles, dot and dash amplitude modulated, and the otherof 50.05 kilocycles, also dotV and dash Yamplitude modulated, with the modulation 180 degrees out of phase. In either case similar` currents will occur in the receiver and the principles of frequency modulation signal reception maybe applied to supplement the principles of widebandv artificial frequency modulation in o producing high grade selectivity.

The A and B oscillators I2 and I 4 jointly feed the outputs thereof into the push pull modulator I5 so that two continuous wave equivalentsare produced of frequencies equal t0 the sum andthe Vdifference of the instantaneous Values cf theA and B oscillator frequencies.y This energy may then be passed through a power amplifier i6 and a following suitable radio frequency band pass filter I'I to create the transmission spectrum indicated in Figure 11; the resultant energy is radiated from the'antenna I8. t

.I'his radiation is seen, from Fig. 1l, to 'cover a total band width of 200,000cycles centered at 50 megacycles, the energy being substantially uni- Y formly distributed throughout the band. Half of the total band width is due to-therdifference ofthe two continuous wave equivalent frequen- Y cies, namely twice thev instantaneous frequency of the B oscillator. The remainder of the total'band width is due to the artificial Variation of the frequency of the A oscillator.. If the transmitted spectrum is directly, or after'frequency conversion, impressed upon an amplitude modulation detector, currents corresponding to twice the frequency modulated B oscillator will bevproduced by the heterodyning action of one of the continuouswave equivalents upon the other.

`A Vreceiver for receiving transmitted energy of this type is showny diagrammatically in Figure 2, and the receiver is constructed in accordance with the teachings of U. S. Patents Nos. 1,522,882 and 1,681,293 granted to JohnV Hays Hammond, Jr. on

January 13, 1925 and August 21, 1928 respectively, Such a receiver may comprise a rst tuner connected to an antenna 2|, a first detector 22 which is connected to a frequency changer oscillator 23; a second tuner 24; a second detector 25;

a third 'tuner 26; third detector 21, the latterV being connected to a heterodyne oscillator 28. The output of detector 21 is fed to a 1000y cycle filter 29, the output circuit of which is connected to a reproducer, such as a pair of head phones 30. i

In thisreceiver system the first tuner 2U, first detector 22, frequency changer oscillator 23, second tuner 24 and second detector 25 function to produce a current of a frequency equal to the instantaneous difference ofthe two continuous wave equivalents, namely 99.9 or 100.1 kilocycles. SuchY currents are sharply tuned by the third tuner 25 to discriminate against all differences between disturbing continuous wave vequivalents which are outside this range. Y The heterodyne Voscillator 28 and third detector 2l constitute a current detector for producing audio tones, such as for-example, 800 cycles at oneY end of the key modulation ofthe B oscillator with the key up and 1000-l cycles at the other end with the key down. For further reducing the amount of interference that othertransmitters can produce a special type of circuit utilizing vboth the 800 and 1009 cycle currents is provided. fA brief analysis of the nature of such disturbances is given below.

` Suppose, for example, that in the transmitter depicted in Figure 1 the A wobbler I I was inoperative and that the A oscillator is exactly 50 megacycles, then with the key down the continuous wave radiations would be 49,950,059 and 50,049,950

cycles, as indicated at C'1 and C z respectively in Figure 3, thereby producing a detected output of 99.9 kilocycles. With the key up slightly different radiations would be produced yielding 100.1 kilocycle detected output. If these radiations were observed, and a standard continuous wave transmitter caused to radiate, an interfering frequency of 50,049,950 cycles would be produced as indicated at I exactly matching one of the transmitted radiations withthe key down.

Under these conditions the third tuner 26 and third detector 21 would beactuated not only by the conjoint action of C1 and Cz, but, also, by the conjoint action of C; and I, so that communication would be disrupted if, for example, I is about 6 decibels below C2 in strength. If, however, the A wobbler II is operative so that Crand VC2 are varied by about 50,000 cyclesjfrom mean, then the beatnote Vbetween C1 and C2 remains 99.9 kilocycles as before, independent of the Vstate of wobble, while the beat note d -ue to Ci and I is frequency modulated between the approximate limits of 50,000 `and 150,000 cycles.VV 4,Under these conditions the third tuner 26 with a* band width of the order of 400 cycles is efficiently actuated by the desired signal, but very ineffectually interfered with by thev frequency modulated signal resulting from the heterodyning of C1 and I.

The output ofthe second detector 25 is illustrated in Figure 4 andshows the signal energy either C1 (99.9 kilocycles with the key down) or C2 (100.1 kilocycles with the key up) and the interference I covering theY wide bandfrequency modulated energy range from 50 to 150 kilocycles If, for example, C2 and. I are equal andthe desired signal in the output of the second detector 25 isV 1 microwatt, then the interfering signal is 1 100 of a microwattper kilocycle of band width. Hence, withA a band width of 41u00.Y cycles, suffi-,-

ciently broad for the desired signal, the interference is only 0.4 of onepercent as strong in the third tunerQ26 as it would be if the A wobbler were inoperative. ASince the signal is independent of the wobble, the 4action of the wobbler. reduces the interference by about 24 decibels.

Further reduction of interference maybe accomplished by taking advantage .of the fact that the third tuner 26 is energized with constant amplitude signals regardless of whether the key is `up or downfand could be provided wwith a lim,.-

with respectV to the signal iter fdevlce for improving `the l'ratio vof 'signal'to interference, or it could be provided with back a to back detection. T-he 'present vinventioncovers the case with the energy converted down to .a lower frequency, for example to the Iaudio band, by use of a suitable heterodyne detector, making the audio signal either 800 or 1000 cycles depending upon the vposition of the key.

A typical audio `frequency discrim'ina'tor is depicted in Figure 5. In this figure the heterodyne detector tube of the lthird detector 21 :of Figure I2 is indicated at L35. The plate circuit .of the tube .35 feeds, through a coupled circuit net- Work 36 "designed rior maximum transmission :at 800 and at 1000 rcycles,.an:amplier tube .31. 'The plate circuit of the tube 31 is connected by .apotentiometer v38 to two `filter circuits 39 and 40. The latter lare tuned to 800 .and v1000 cyclesrespectively, `and are each .connected `:to the respective input electrodes of two tubes 4i and 42. The output circuits of the tubes 4I and 42 are :coupled 'by two transformers 45 and 4B totwo rectiers 41 and 48. A center point galvanometer 49 'may be connected between the ycathodes ofthe rectiiier tubes 41 and 48. The galvanometer 49 may |be replaced :by Aa relay or other device to control .a

vsound type indicator, such as thehead phones 30 of Figure '2.

The output ofthe heterodyne detector I35, carrying the 800 and 1000 cycle currents and the disturbance, feeds through lthe coupled circuit l36 and is -amrifled :by the tube 31. The output energy :from the tube 31 passes through the potentiometer 38, and is split up by the two lter circuits 39and A40 so that the input grid of the tube l 4l will be energized at a maximum with 800 cy cle energy 'and the grid .of the tube 42 will be yenergized at a maximum vwith 1000 cycle energy. The tube 4I will, therefore, be most strongly energized with 'the key up, Aand the tube 4:2 willbe most' strongly 'energized with the .key down.

The ycircuit may be so adjusted by .the potentiometer V38 that the band width of transmission from the plate of :the tube 35 to the tube 4:| is the same as to the tube 42, so that equal Isignals of 800 and 1-000 cycles are produced. Figure 4 indicates that during a period of several cycles of Awobble of the A oscillator substantially `as much 'interference or stray energy will be impressed upon the 'grid of the tube 4l as u pon the -grid of the tube 442, regardless of lthe position of the key. The 800 cycle circuit 39 will be energized to a greater extent with the key up, and the 1000 cycle circuit 40 will be energized to va. greater extent with the .key down.

Vifhichl circuit is energized to the greater extent and, therefore, Whether the :key is up or down, may bedetermined `by the use of the rectiers 41 and 48. In `this system the lampliiied energy from the tube 4i drives zthe rectier 41 through the vtransformer 45, and the amplified energy from the tube 42 drives the rectifier 48 through the transformer 46. .A comparison :of the rectiiied .outputs may .be made by the `center point galvanometer 49, which is connected between v'the lcathodes of the rectiiiers 41 and 48. The load resistors of the rectifier-s are shown arranged in .series between the cathodes of the rectifiers; the junction of the .series resistor is grounded. The Vgalvanometer 49 may, if desired, be `replaced by a relay to control :a sound type indicator. By `balancing the ysystem carefully -so that 'the rectiiiers 41 zand 48 produce .similar effects considerable improvement in selectivity over a tuned ycircuit can be obtained.

The sensitivity or the indication, however,- vis not independent of the presence or absence of the interference, as it `decreases `with fthe "increase of interference. This 4is apparent from the `following analysis vin which Er represents the random energy, which, although `not .fof the same wave is equally impressed upon the two rectiiiers 41 and 48 when averaged foverFone-or two cycles of the A wobble. ,En is the energy operating the -rectier 41 when the key is up, and Ea :is lthe energy operating the rectifier 48 when the `key is down. The signal energy Es=EdEu.

With the key up or ldown the numerical magnitude ofthe difference :of the potentials V of the cathodes of the rectifiers v4?! and 48 is: y

in which K Ais a constant yof the :system` is :the ratio of the stray energy to the signal energy, then the relative ysensitivity R given by:

The 'following table indicates the changed! re1- ative sensitivity as the ratio of interference to signal is increased: i if Erl I E.

When the interference in the y1000 cycle circuit :reaches about 20 percent of the signal on an energy basis vthe circuit of Figure 2 .becomes lun satisfactory. lOne .method of compensating `for loss of sensitivity for the desired'signal as .the ratio of interference to vsig-nalincreases is shown in Figure 6. .'Ihis method .determines the -eiect in accordance with the sum of the rectied .currents vas well as theirdi-fference.

In `Figure 6 the secondaries ofthe .transformers 45 a.nd:46 are connected to the opposed rectiers 41 `and 48 .in a manner ,similar to that shown vin Figure 5. Theroutputfcircuitsof the Erectifiers 41 vand 48 `areconnected to adynamometer type .relay 5i which is provided with ,a .rotor coil 52 and a stator V.coil 53. 1

The recti-iied currents .from ,the output circuits of the rectiters 41 and 48 in IFig. -6 .will pass through the rotor winding 52 in opposite directions. and through the stator winding 53 in .the

same direction, .as indicated by the arrows radis cent :the .respective windings. Thus, the current flowing through the rotor winding 52 `is propertional tothe diierence of the two frectied -currents,and .the current vthrough .the :stator winding l.53 is proportional `to the .sum `of :the 4tworectified currents. The torque of the dynamometer 9.5i vis proportional vthe 4prin-inlet -of the magnetizing eiects yof .the-:currents 'in the ,rotor winding 52 @and .the stator winding The magnetizing .eiectfor the vrotor vfindin-gy i52 will be: f

#CMM-:cama l In Figure7 is shown another method of comf pensation bythe useof direct current amplifying circuits using tubes witha special grid structure forwhich the plate current is a suitably' increasing function of the grid voltage. Tubes of the 6K7 type, with screen and suppressor connected to the plate, have approximatelythe proper charr acteristics. lIn Figure '7 the secondaries of the transformers 45 and 46 are connected to the respective rectiers 41' and 48.V VThe output circuits of the latter `include series-related.resistors 55 and 56, which Ain turn are connected to the input grids of'twoY direct current amplifying tubes 51 and 58.v The latter may be of the special grid structure type previously referred to. The input-grid :of tube l58 is connected to the cathode end of load resistor 56, while the grid yof tube 51 is connected to the cathode end of resistor 55. The junction of resistors 56 and 55 is connected -to the negative terminal of the grid bias source of tubes.,58-51. The output circuits of thetubes 51'and 58'are connected to a resistance network YSii-,60. Y The Alatter isv bridged by an indicating device 6|, such as a center point galvanometer.

vThe rectified output current of the rectifier tubes4'1 and 48, in Fig. '1,'ows1through-the resistors and 56 causing a potential difference to, be built up across these resistors which is proportional to the current passing through them.' This potential is impressed upon the grids of the tubes 51 and 58, and controls the plate current as an increasing function of the grid voltage. The plate current Vof. the tubes 51 and 58 passes Vthrough the'resistance network 59-60 in the direction of the'rv arrows towards the respective plates of theltubes,uand Acauses a potential difference tov be built up across ,these resistors proportional to the current vflowing them. vThe gal- Vvanometer 6| indicates the potential difference across the resistance network 59--60, and, therefore', the relative strength of the signals inthe 800` and 1000 c yclesV channels, thus indicating Whether the key is up or down. In this system the interference raises the average of the lgrid voltages of the two tubes 51 and 58 so that smaller difference of the rectified current due to the'inte'r- Vference producessubstantially a iixed amountjof change of plate current in the resistors 59 and 60. This does not affect the indication of the gal- Y vanorneter 6I.

Figure 8 shows an expander type compensator used in connection with the circuits shown in Figure 5. In' this v'system the secondaries of the transformers 45 and 46 areconnected to the rectiiiers 41 and 48. lBetween the cathodes ofthe rectiiiers is bridgedV a resistor 65 shunted by `a condenser'66. `The Vresistor 65 is in shunt across the opposed diode resistors. The input electrodes of a-tube`61 are connected across the resistor 65. The grid circuit of tube 61 includes the secondary of a transformer 68, the primary of which'is connected to atone'sour'ce 69. The latter Ymay be an alternator generating audio frequency current.

The output electrodes ofthe tube 61 are connected through atransforrner 1|) to the input circuit of tube 1| which forms a part of an expander type amplifier vv12. Connected in the grid return path of the tube 61 is aresistor 13, one end of which is connected to the midpoint of the resistor 65, and the other end is grounded. The

fio

resistor 13 is,- also, included in the grid circuit of tube 1|. The output circuit of the tube 1I is connected through a transformer 15 to a pair of headphones 16. o

In the operation of the system shown vin Figure 8, the rectified currents from the rectiiiers 41 and 48 flow through the resistor 65 in the` direc- Y this resistor which is proportional to the current therethrough. Thispotential controls the gain of the amplifier 12, which is, therefore,controlled by the sum of'two rectified rectifiers 41 and 48.v

Thus, when the key is up and there is more energy passing through the transformer 45 from the '-800 cyclecircuit than there'is through the transformer 46 from the 1000 cycle circuit, more current will pass through the lower-half ofthe resistorV than through Athe upper half. AThis will cause a decrease in the gain of the amplifier 61 so that very little of the tonal'current from the tone source 69 will be transmitted to the head phones 16. On the other hand when the key is down more current will pass through vthe upper half of the resistor 65 thus increasing the gain of the amplifier', which will increasethe transmissionv of the tonal current to the headphones 16 thereby giving'an audible signal.

The change of rectified currents'is 'modified by the presence of interference in both channels when the signal changes from the 800 to the 1000 cycle channel. The production'of tone by the Akeying tube 61 diminishes as'the interference may be inserted in the grid circuit of the tube 61 shown in Figure 8. In this modified form of the invention two resistors 18 and 19 are connected in the grid circuit of the tube 61, and a rectifier tube is connected to the junction of these two resistors. In the operation of the circuit shown in Figure 9, the Ytube 80 actsfto limit the potential applied to the grid of the tube 61 to a deiinite predetermined value. Hence, when no interference is present, and a high potential is produced across the resistor 65, the potential on'the grid of the tube 61 will be held to a defi nite limit. As the interference increases the potential across the resistorV 65 will decrease. As long as it remains higher than the predeterminedyalue the potential on the grid of the tube 61 will remain unchanged, thus making the output of the keying tube' 61 more independent of the interference value. 1

currents from vthe and 83. The resistors 8l and 82 are shunted respectively by condensers 85 and 86. The resistors 48 and 41 are connected in series between the diode cathodes, each resistor being properly bypassed. The junction of the two resistors 48 and 41' is grounded. 'Ihe resistors 82 and 8| are respectively in series with resistors 48' and 41' in the respective space current paths of diodes 48 and 41. The resistor 83 is common to both resistors 82 and 8i, and is connected between the anode ends thereof.

In the operation of the automatic volume control arrangement shown in Figure 10, the rectified currents from the respective cathodes of rectiers 41 and 48 flow to respective anodes through the resistors 8| and 82 in the direction of the arrows thereby causing negative potentials to be built up at the resistor 83 which are proportional to the rectified currents. The negative potential of the midpoint of the resistor 83 is used as the automatic volume control bias in a manner well known in the art which need not be more fully described herein. Itis sufficient to point out that such bias is applied over the lead from the midpoint of resistor 83 to prior amplifiers of the system.

While the invention herein is illustratively practiced with respect to improvement in the previous mentioned patents, it is not intended to be in any way limited to any particular transmission system. For example, it may well be applied to continuous wave transmission systems with the wave not keyed off and on, but with the wave shifted between two somewhat different values, or to any frequency modulation system falling within the scope of the appended claims.

Although only a few of the various forms in which this invention may be embodied have been shown herein, it is to be understood that the invention is not limited to any specific construction but might be embodied in various forms without departing from the spirit of the inveny tion or the scope of the appended claims.

What is claimed is:

l. A frequency modulation transmission channel provided with an indicator, a receiver including frequency modulation detecting means comprising a pair of rectiiiers and a differential output, and means interposed between the rectifiers and indicator to make the indication substantially independent of interfering disturbances upon both rectiers, said' means including a first device controlled in accordance with the difference of the outputs of the rectiers, means to vary the operation of the first device at a periodic rate, and a second device responsive to the rst device and controlled in accordance with the sum of outputs of the rectifiers.

2. A frequency modulation transmission channel provided with an indicator, a receiver including frequency modulation detecting means comprising a pair of rectiers and a differential output, and means interposed between the rectiers and indicator to make the indication substantially independent of interfering disturbances upon both rectiers, said means including a first device controlled 'in accordance with the dierence of the outputs of the rectiers, and a second device controlled in accordance with the sum of outputs of the rectifiers, said first device being a keying circuit, and the second device being a variable gain amplifier for the output of the keying cir` cuit.

3. In na signalling system wherein there is transmitted to a receiver waves composed of the sum and difference frequencies of a high frequency Wave and a lower frequency wave whose frequency has been varied to either side of the lower frequency, means receiving the transmitted frequencies and deriving therefrom signals corresponding to said signalling variations, a frequency-variation responsivedevice having an input network to which said derived signals are applied, and means actuated by the responsive device for indicating solely said derived signals.

4. In a signalling system wherein there is transmitted to a receiver waves composed of the sum and difference frequencies of a high frequency wave subjected to a saw tooth frequency sweep and a lower frequency wave whose frequency has been varied at an audio rate to either side of the lower frequency, means receiving the transmitted frequencies and deriving therefrom audio signals corresponding to said signalling variations, a frequency-variation responsive device having an input network to which said derived signals are applied, and means actuated by the responsive device for indicating solely said derived signals.

ELLISON S. PURINGTON. 

